Optimize storage of tiny strings (up to 3 characters)

extras/tests/MsgPackDeserializer/deserializeArray.cpp

@@ -5,8 +5,11 @@
 #include <ArduinoJson.h>
 #include <catch.hpp>
 
+#include "Allocators.hpp"
+
 TEST_CASE("deserialize MsgPack array") {
-  JsonDocument doc;
+  SpyingAllocator spy;
+  JsonDocument doc(&spy);
 
   SECTION("fixarray") {
     SECTION("empty") {
@@ -30,6 +33,24 @@ TEST_CASE("deserialize MsgPack array") {
       REQUIRE(array[0] == 1);
       REQUIRE(array[1] == 2);
     }
+
+    SECTION("tiny strings") {
+      DeserializationError error =
+          deserializeMsgPack(doc, "\x92\xA3xxx\xA3yyy");
+
+      REQUIRE(error == DeserializationError::Ok);
+      REQUIRE(doc.is<JsonArray>());
+      REQUIRE(doc.size() == 2);
+      REQUIRE(doc[0] == "xxx");
+      REQUIRE(doc[1] == "yyy");
+      REQUIRE(spy.log() == AllocatorLog{
+                               Allocate(sizeofPool()),
+                               Allocate(sizeofString("xxx")),
+                               // Buffer is reused for the next string
+                               Deallocate(sizeofString("xxx")),
+                               Reallocate(sizeofPool(), sizeofPool(2)),
+                           });
+    }
   }
 
   SECTION("array 16") {